1. Field of the Invention
The present invention relates to an engine exhaust gas recirculation system effective to reduce the emission of nitrogen oxides (NO.sub.x).
2. Description of the Prior Art
The prior art engine exhaust gas recirculation systems (hereunder called "E.G.R. systems") conventionally used to reduce NO.sub.x emission are classified into plate type and manifold type. The E.G.R system of the plate type includes an E.G.R. passage extending from an exhaust pipe of an engine to an intake pipe thereof between a carburetor venturi and a throttle valve. A fixed restriction orifice is provided in the E.G.R. passage. The recirculation of engine exhaust gases according to this type of E.G.R. system is a function of the engine back pressure. In the manifold type E.G.R. system, an E.G.R. passage having a restriction provided therein extends from the exhaust pipe to the intake manifold downstream of a throttle valve.
E.G.R. systems of both types are generally called "external E.G.R.". In contrast, it has been found that residual gases retained in combustion chambers of an engine are also effective to reduce the NO.sub.x emission. Thus, to retain a part of combustion gases in the combustion chambers is called "internal E.G.R.". Thus, the total amount of exhaust gases obtained by the external and internal E.G.R.s must be taken into consideration.
In order to most efficiently reduce the NO.sub.x emission, it is desirable to keep constant the ratio of the total amount of exhaust gases obtained by the external and internal E.G.R.s to the amount of engine intake air (i.e., total E.G.R. rate). It has been found that the rate of the internal E.G.R. is increased and decreased as the load on the engine is decreased and increased, respectively. Accordingly, in order to keep constant the total E.G.R. rate, the external E.G.R. system must be controlled such that the external E.G.R. rate is increased and decreased as the engine load is increased and decreased, respectively.
The plate type E.G.R. system of the prior art has an advantage that the amount of the recirculated exhaust gases is proportional to the amount of the engine intake air so that the total E.G.R. rate obtained is substantially close to the desirable total E.G.R. rate. The E.G.R. system of this type, however, has disadvantageous problems that a deposit of a foreign material is formed on the throttle valve, the carburetor suffers from themral influence, icing occurs at a low temperature, and so on.
In the manifold type E.G.R. systems, the disadvantageous problems of the plate type E.G.R. system hardly occur because exhaust gases are recirculated directly into the intake manifold downstream of the throttle valve. In the manifold type E.G.R. systems, however, the recirculation of exhaust gases is controlled by pressure signals obtained from the venturi vacuum or intake manifold vacuum. Thus, the amount of recirculated exhaust gases is dependent on the pressure difference between the engine back pressure and the intake vacuum and on the exhaust gas-flow cross-sectional area of the restriction orifice in the E.G.R. passage. The amount of recirculated exhaust gases is influenced particularly by the intake vacuum, so that the external E.G.R. rate is increased and decreased as the engine load is decreased and increased, respectively. This external E.G.R. control characteristic is opposite to the desirable external E.G.R. control with resultant occurence of surging and misfires at a light load engine operating condition and thus with decrease in the reduction of NO.sub.x emission.